Given,
time taken to reach top(t)=5s
As the object is thrown up, final velocity is zero. (v=0)
The acceleration due to gravity will be negative as the direction of motion is opposite to the direction of gravity.
Using equation of motion,
v=u+gt
⇒u=v-gt
Putting the values,
u=0-(-10)(5)
⇒u=50 ms^-1